Transcranial magnetic stimulation (TMS) is a noninvasive technique used to apply brief magnetic pulses to the brain, or to other human organs, and to thereby activate neuronal structures. The pulses are administered by passing high currents by a stimulator through an electromagnetic coil externally placed upon the patient (for example, placed on the scalp for brain treatment), inducing electrical currents in the underlying tissue, thereby producing a localized axonal depolarization. This technique has become a major tool in central nervous system research, as well as a potentially promising treatment option for various neurobehavioral and neurological disorders.
In most TMS devices, a capacitor is charged to a pre-defined voltage, and the magnetic stimulation is performed by discharging the capacitor through a single stimulating coil, using a single fast switch. The brief current in the TMS coil induces an electric field proportional to the time derivative of the current. When applied to neuronal tissue, the electric field may lead to a change in the neuronal trans-membrane potential. This potential change may result in either hyper-polarization or depolarization of the membrane. When the membrane is depolarized to a critical level, then under certain conditions a neuronal stimulation will occur.
The current pulse shape produced in conventional TMS devices is either monophasic or biphasic sinusoidal. The pulse shape is determined by the capacitance of the capacitor C, the stimulating coil inductance L, and the resistance R in the circuit. In stimulators with biphasic pulses—unlike with monophasic pulses—part of the energy returns to the capacitor at the end of a cycle, enabling repetitive operation. Hence, biphasic pulses are used in repetitive TMS (rTMS), while monophasic pulses are usually used to produce single pulses.
A method termed controllable TMS (cTMS) has been disclosed in Peterchev et al.: A Transcranial Magnetic Stimulator Inducing Near-Rectangular Pulses With Controllable Pulse Width (cTMS), IEEE Trans Biomed Eng 2008, 55:257-266. In this method, an insulated gate bipolar transistor (IGBT) is used as a switch, and a monophasic pulse can be truncated in a controlled way by turning off the IGBT. Energy from the coil is dissipated in a resistor and is not returned to the capacitor; hence the ability for repetitive TMS in this method is limited. The extension of this method to a biphasic pulse shape is disclosed in US Patent Publication Number 2007/0293916 A1. This is done by using two capacitors and IGBTs for the two phases. However, in this method the switching is usually performed while the current is high, which may lead to serious problems of transient voltage spikes and switching losses. Moreover, this disclosure is limited to producing rectangular pulse shapes only. In addition, in this disclosure there is no possibility of inducing different pulse shapes in different body organs or body organ regions.
The exact neuronal tissue response may depend on the specific pulse parameters, such as pulse shape, of the induced electric field. Thus, it would be advantageous to have a method to induce variations in pulse parameters, such as pulse shape, in a controlled way. Increased variability and flexibility in control of pulse shape parameters may be useful for brain research as well as for various clinical applications in psychiatry, neurology and disorders related to peripheral nerves.
A method of multi-channel transcranial magnetic stimulation was disclosed in US Patent Publication Number US20060287566 In this method, different coil elements are operated using separate channels. The multiple channels may be activated simultaneously, or sequentially with different delay times. The time delays between the operation of each channel are controlled on a level of microseconds. This disclosure enables one to obtain various pulse shapes, but is limited to combinations of biphasic and monophasic pulses.